This invention relates to the broad field of radar counter measure devices, and more particularly to a system of at least one decoy radar transmitter, deployed away from and linked to a main tactical radar complex, for protecting said radar complex against attack from an armed radar homing missile by providing radar emissions which are phase coherent to the transmission of the radar complex.
Tactical stationary radar facilities are normally deployed at locations to provide surveillance of a portion of airspace to detect moving targets. These radar complexes usually employ a parabolic reflector type main antenna which generally comprises some predetermined array of feedhorns. The object of the antenna is to direct a pulsed pencil beam of radiation, known more commonly as the main lobe of radiation, in desired directions normally fanning the airspace. In addition to the main lobe, the radar antenna also transmits other unwanted radiations, known as side lobes, which result principally because of imperfections in the radar antenna design.
Radar radiation reflections from objects within the conveyed airspace are received by the radar receiver and are generally correlated by a moving target indicator (MTI) type processor which is generally incorporated as part of the radar receiver. The MTI usually is equipped with a signal conditioning and filtering processor which utilizes the phase information from the reflected pulse radiations to determine valid moving targets from existing environmental clutter. In operation, the radar transmitter sends out pulsed energy in an aimed direction at a predetermined pulsed repetition frequency (PRF). Reflections may be returned from a number of objects in each aimed direction within a known time portion (target return time) of each period of the PRF. The filter signal processor of the MTI normally compares the phase of successive reflected pulsed radiation information to discriminate moving targets from existing clutter. Returned pulse radiation which has little or no phase differences between successive reflections is usually considered as clutter and is rejected from other returned pulse radiation which bear substantial phase differences. The reflected radiation information passed by the MTI filter processor is considered as potential moving targets.
Recently, specific attack weaponry, known commonly as anti-radiation missiles (ARM), have been designed to home in on the side lobe radiation transmitted by the radar antenna for the purposes of destroying the radar facility. Some of these ARM's use broadband tuned receivers adjusted to radar frequencies as a means of direction tracking. Their guidance systems are known to direct the missile trajectory normal to detected wavefronts of the side lobe radiation generated from the main radar antenna, whereby the ARM may be automatically steered to the center of the radar antenna. These type ARM's are limited in their guidance dynamics by the response of their servodynamic mechanisms which is primarily interested in direction steering the ARM in the path toward the source of side lobe radiation.
One proposed method of counter measure against an ARM attack is the strategic deployment of one or more small radar transmitters, known as decoys, away from the main radar site. The purpose of these decoys is to transmit signals which attempt to imitate the main radar side lobe emissions to confuse an ARM that may be attacking a radar complex and cause the attacking ARM to impact harmlessly at a point away from both the main radar complex and the decoys. More specifically, the decoy transmissions are designed to combine in space with the main radar side lobe transmissions to compositely form wavefronts which appear to the guidance system of an ARM as being transmitted from a virtual side lobe radiation source remote from either the main radar complex or the decoy. These compositely formed wavefronts maintain confusion within the ARM's guidance system until the radar transmission wavefronts of either the main radar or decoys are singularly detected by the ARM; but by this time, it is estimated that the guidance mechanisms of the ARM are unable to respond to redirect the ARM away from the designated impact point set up by the compositely formed wavefronts. In order to confuse the ARM in the aforementioned manner, the decoys must approximate the main radar side lobe emissions as closely as possible. However, if the side lobe radiation is not emulated properly by the decoy, it may in addition to confusing the ARM also confuse the MTI function of the main radar. One likely possibility is in the case when the decoy signals are reflected off of stationary clutter and are received by the radar MTI processor during the target return time.
The side lobe radiation of some known proposed types of decoys are triggered off of the main transmission pulse of the radar which provides for time synchronizations of the side lobe radiation of the decoy with the main radar side lobe radiation, but does not provide for any phase coherent relationship therewith. Reflections off of stationary clutter from the side lobe radiation of these types of decoys may appear to the MTI portion of the radar receiver as if the Earth is shaking back and forth, in which case, all such clutter may have apparent doppler effects and be passed through the MTI filter processor. Thus, the side lobe radiation from these types of decoys may destroy the filter processing of the MTI, under certain conditions, by producing false doppler phase changing effects causing stationary clutter to be falsely identified as moving targets. Accordingly, if decoys of this type are used as a counter measure against attacking ARM's, the MTI processor of the main radar may not be capable of nulling out clutter properly under all conditions and in some cases, it may be difficult to distinguish an actual moving target from unfiltered clutter. Therefore, it appears that if decoys are to be a viable counter measure against ARM's for protection of a radar complex, the imitation of the main radar side lobe emissions by the decoy should be enhanced to the point where reflections from clutter will not interfere with the MTI processor's rejection of unwanted clutter.
In another aspect of main radar transmissions, the high frequency carrier waveform within the pulsed transmissions of the main radar are subjected, at times, to certain coded phase reversal patterns, like Barker codes, for example. Previously proposed decoy systems are not known to emulate main radar side lobe radiation to this extent. It may be possible, under some conditions, that the guidance system of an ARM may be capable of distinguishing the different phase patterns between the main radar and decoy side lobe radiation. To improve upon the counter measure protection provided by a decoy system, a better replica of the main side lobe radiation which includes these phase reversal patterns as provided by the decoy transmissions should be considered.
In addition, most known previously proposed decoys are to be individually provided with operating power away from the radar by means of an engine-generator set. In the case in which a plurality of decoys are deployed about the radar complex, each would require its own engine-generator set and associated fuel storage capabilities. Such proposed decoy installations have been considered relatively expensive and heavily burdened with mechanical apparatus which may tend to reduce their availability. Any improvement in decoy systems which would simplify the method of providing operating power hereto would surely enhance the probability of their becoming an integral part of all tactical radar complexes in the future.